The entrypoint build script is gcp_ubuntu/build.sh.

To manually trigger the unit test run as the CI system does:

export GIT_REPO_DIR=<absolute path to the git repo>
export KOKORO_ARTIFACTS_DIR="${GIT_REPO_DIR}"
ci/kokoro/gcp_ubuntu/build.sh

That is, if the current directory is the root of the Git repository:

GIT_REPO_DIR="`pwd`" KOKORO_ARTIFACTS_DIR="`pwd`" ci/kokoro/gcp_ubuntu/build.sh